I bought a few of the large containers, but shaped differently than those. Tapered base with a small knob on top. Called isotop containers.Weighed about 32 lbs apiece, with a small brass threaded nut cast into them. Alloy was 1%Sn, 3%Sb, the rest pure lead.
Some of the smaller variety are supposed to run 2.5%Sn,2.5%$Sb.
Easiest way to find out is have them tested with an XRF gun at a salvage yard. Some places will test for free if you're buying or selling something, others have a fee. Not all are calibrated that well though.
